Thx a lot for your answer.
I finally managed this way:
CREATE MEMBER CURRENTCUBE.[Measures].[Full Year]
AS AGGREGATE(
ANCESTOR( [Time].[Year - Month].CurrentMember,[Time].[Year - Month].[Year] ), [Measures].[Value]),
FORMAT_STRING = "#,##0.00;-#,##0.00",
VISIBLE = 1 , DISPLAY_FOLDER = 'Calculated measures' , ASSOCIATED_MEASURE_GROUP = 'Line Items' ;